Signal light noise reduciton apparatus and signal light noise reduction method

ABSTRACT

An increase in the span of the transmission distance is aimed at by reducing unwanted ASE generated during optical communication. A carbon nanotube is employed as a saturable absorber  15  and this saturable absorber constitutes a noise reduction apparatus that has the function of cutting off or reducing transmission of unwanted ASE or the like which is of weak signal light intensity and of allowing transmission of signal light of strong light intensity. This noise reduction apparatus is arranged for example in the transmission path of signal light of a bidirectional excitation type EDFA, more precisely the apparatus is inserted in the latter stage of the EDF  40.  In this way, carbon nanotubes having a saturable absorption function can be utilized in the field of optical communication.

TECHNICAL FIELD

The present invention relates to apparatus for reducing noise of signallight in optical communication (hereinbelow referred to as noisereduction apparatus).

BACKGROUND ART

With the enormous progress in optical communication technology in recentyears, further increase in the span (increase in distance) of thetransmission distance of signal light is desired.

A method that is currently adopted with a view to increasing the span ofthe transmission distance consists in compensating for the attenuationof signal light intensity that accompanies transmission distance byrelaying the signal light through a transmission medium such as anoptical fiber by a plurality of optical amplifiers that perform opticalamplification of the signal light.

Optical fiber amplifiers that have attracted attention in recent yearsinclude erbium-doped fiber amplifiers (hereinbelow referred to as EDFAs)that make use of the phenomenon of stimulated emission using erbiumexciting light.

Because of excellent matching with the transmission medium, opticalfiber amplifiers are suitable for employment in optical transmissionsystems; EDFAs are particularly suitable since high gain and highefficiency can be achieved, due to matching of the amplificationwavelength zone with the very low loss wavelength band of a quartz fiberin the 1500 nm wavelength band.

However, in the case of optical amplifiers such as, in particular EDFAs,amplified signal light is generated by a population inversion producedby the exciting ions. In the amplification step of this signal light,spontaneous emission that is randomly generated is also amplified, soamplified spontaneous emission (hereinbelow referred to as ASE) i.e.optical noise (also referred to as noise) is generated from the opticalamplifier.

Since ASE having a random phase is added to the amplified signal light,the result is that the ratio of signal light to optical noise (S/Nratio) is severely affected.

Due to the admixture of ASE, not only is it not possible to output onlyprescribed signal light with high accuracy from the optical amplifier,but also the ASE undergoes repeated optical amplification duringpropagation through the optical fiber and other members in the same wayas the signal light.

As a result, this undesired ASE that is generated presents aconsiderable obstacle to increasing the span of the transmissiondistance.

DISCLOSURE OF THE INVENTION

Discovery of technical means for solving the above problems wastherefore desired.

With this in view, first of all, the inventors of the presentapplication conducted meticulous research focusing on the fact that,normally, the light intensity of the optical noise when initiallygenerated is fairly small in comparison with the light intensity of thesignal light. As a result, they discovered that it was possible toamplify exclusively the signal light and to suppress amplification ofoptical noise, by utilizing the characteristic possessed by a carbonnanotube saturable absorber, namely, that its absorption decreases withthe square of the optical power, resulting in an abrupt increase intransmissivity, and that it was therefore possible to allow thepropagation only of signal light and to cut off optical noise.

Accordingly, noise reduction apparatus of signal light according to thepresent invention has the following structural characteristics.

Specifically, this noise reduction apparatus is constructed using acarbon nanotube as the saturable absorber. This noise reductionapparatus may be arranged in the transmission path of the signal lightin order to reduce signal light noise in optical communication.

With such a construction, the carbon nanotube constituting the saturableabsorber cuts off transmission of for example ASE of weak lightintensity and, in addition, transmits a signal of strong light intensityand so is capable of reducing signal light noise.

Preferably, also, the carbon nanotube may have optical non-linearity.

Preferably, also, by combination with an optical amplifier, thesaturable absorber has a function as an optical isolator in respect oflight propagated in the opposite direction to the signal light.

Light propagated in the opposite direction to the signal light may befor example reflected light of the signal light. The light intensity ofthe reflected light is weaker than the light intensity of the signallight. Consequently, furthermore, the saturable absorber can be made tofunction as an optical isolator of the signal light and the reflectedlight, making it possible to achieve a straightforward deviceconstruction for optical communication.

Preferably, also, the saturable absorber has the function of a waveformshaper in respect of the signal light. Since, in the intensitydistribution of the signal light of the saturable absorber, portions ofweak light intensity can be cut off and portions of strong light.intensity can be transmitted, the pulse waveform of the signal lightthat is transmitted through the saturable absorber can be shaped to asteep waveform.

Preferably, also, the wavelength zone of the saturable absorber that iscapable of saturable absorption is at least 1200 nm but no more than2000 nm.

In this way, matching can be achieved with the wavelength band of thesignal light, whose transmission medium is for example a quartz opticalfiber as currently employed.

Preferably, also, the signal light is signal light that is emitted froman optical fiber amplifier.

Preferably, also, the optical fiber amplifier is an erbium-doped opticalfiber amplifier.

In this way, practical utility can be achieved by enabling matching ofthe very low loss wavelength band of a quartz optical fiber in anerbium-doped optical fiber amplifier and the saturable absorptionwavelength zone of a saturable absorber.

Preferably, also, the signal light is signal light emitted from asemiconductor optical amplifier.

Preferably, also, the signal light is signal light emitted from asemiconductor laser.

Preferably, also, if a plurality of optical fiber amplifier stages areprovided consecutively in the transmission path, the saturable absorbermay be provided as a repeater between each adjacent optical fiberamplifier.

Since, in this way, a saturable absorption function in respect of theamplified light emitted from the respective consecutive optical fiberamplifiers can be achieved, this is effective in lengthening the span,because transmission of amplified spontaneous emission can beeffectively cut off.

Preferably, also, for the carbon nanotube, either or both of asingle-wall carbon nanotube or multi-wall carbon nanotube can beemployed.

Preferably, the noise reduction apparatus described above can beconstituted by providing a carbon nanotube on the surface of an opticalcomponent such as a transparent substrate, transparent prism,transparent lens or other component formed by a suitable transparentoptical material. Alternatively, the carbon nanotube may be sandwichedbetween transparent optical materials or may be embedded in transparentoptical material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view given in explanation of the optical absorptioncharacteristic of an SWNT thin film;

FIG. 2 is a view of the absorption band portion appearing in the lowestenergy region in FIG. 1 wherein the horizontal axis is converted tolight wavelength;

FIG. 3 is a view given in explanation of a measurement device for theSWNT thin film, using the Z-scanning method;

FIG. 4 is a view given in explanation of transmissivity at each laserlight intensity, when the SWNT thin film is positioned in the vicinityof 40 mm, in measurement of the SWNT thin film using the Z-scanningmethod;

FIG. 5 is a view given in explanation of an EDFA provided with a signallight noise reduction apparatus according to the present invention;

FIGS. 6(A) to (C) are views given in explanation of the effect ofreducing optical noise produced by an SWNT thin film;

FIG. 7 is a view given in explanation of the waveform shaping effectproduced by an SWNT thin film;

FIGS. 8(A) and (B) are views given in explanation of a modified exampleof the embodiment; and

FIG. 9 is a view given in explanation of the construction of a typicalEDFA.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is described below with referenceto the drawings. It should be noted that in the views employed in thedescription for example the dimensions, shapes and arrangementrelationships of the various structural constituents are only showndiagrammatically such as to enable comprehension of the presentinvention. Accordingly, the present invention is not restricted solelyto the examples illustrated in the drawings.

<1> Verification of the Saturable Absorption Function of the CarbonNanotubes

(1-1) Manufacture of Carbon Nanotubes

In this embodiment, single-wall carbon nanotubes (hereinbelow referredto as SWNT) constituted by tubular structures consisting of a singlesheet of graphen formed by a six-member ring network structure of carbonatoms (C) are employed. However, it should be noted that, for the carbonnanotubes, it would also be possible to employ multi-wall carbonnanotubes (hereinbelow referred to as MWNT) constituted by tubularstructures consisting of multi-layer graphen structures; thus, thepresent invention is not restricted solely to SWNT.

For the manufacture of SWNT, as is generally known, any suitable methodmay be employed such as a laser evaporation method or arc dischargemethod. Hereinbelow, a simple description will be given concerning anexample of a method of manufacturing SWNT using the laser evaporationmethod.

First of all, a composite rod is manufactured containing transitionmetallic elements, for example, cobalt (Co) and nickel (Ni),respectively in the amount of a few atomic% (example 0.6 atomic %respectively) (metal/carbon).

Next, SWNT is manufactured by heating this composite rod to atemperature of about 1200° C. in an electric furnace and theninstantaneously evaporating the carbon and catalyst metal using forexample a neodymium (Nd)/YAG pulsed laser (10 Hz) while introducingargon (Ar) gas at 50 sccm under reduced pressure of 500 Torr. The SWNTthat is thus obtained may be mixed with by-products as impurities and sois preferably refined by any suitable method such as the water heatingmethod, centrifugal separation method or ultrafiltration method.

(1-2) Manufacture of Carbon Nanotubes thin Film

Next, thin film on which SWNT is deposited (hereinbelow referred to asSWNT thin film) is manufactured. For the manufacture of SWNT thin film,preferably SWNT whose diameter is in the range 0.5 nm to 2.0 nm andwhose length is in the range 500 nm to 1000 nm is employed. If thediameter and length of the SWNT are in these ranges, the saturableabsorption effect can be satisfactorily manifested.

In the embodiment described below, SWNT of mean diameter about 1.3 nmand mean length about 1000 nm was therefore employed.

In order to manufacture SWNT thin film, the spray method is adopted ofproducing an SWNT thin film by spray application of a dispersionobtained by dispersing SWNT in a dispersion medium onto a transparentoptical material i.e. a transparent coated material such as a glasssubstrate. A brief description of an example of the manufacture of anSWNT thin film by the spray method is given below. For the glasssubstrate for example a parallel planar plate is employed.

First of all, a dispersion was manufactured by uniformly dispersing SWNTrefined by the method described in (1-1) in a dispersion mediumcomprising at least one of for example alcohol, dichloromethane anddimethyl formaldehyde. If necessary, for example a surfactant may beadded in the preparation of the dispersion. The dispersion concentrationof SWNT is suitably about 1 to 2 mg/ml if for example ethanol isemployed as the dispersion medium. However, it should be. noted that thedispersion concentration is not restricted to this and could be alteredat will in accordance with the object and the design.

The dispersion that was thus prepared is dried by spray application ontoa glass substrate. If the glass substrate onto which spray applicationis performed is at low temperature, the SWNT in the applied dispersioncoagulates with the result that good film properties are not obtained,so spray application is performed while heating the glass substrate.

An excellent SWNT thin film can be obtained by means of the processesdescribed above. It should be noted that the method of manufacturingSWNT thin film is not restricted to this and for example anelectrophoretic deposition method or polymer dispersion method could beemployed.

(1-3) Measurement of the Absorption Spectrum of Carbon Nanotubes

An evaluation of the optical absorption characteristic of the SWNT thinfilm manufactured by the method described in (1-2) was conducted.

SWNT thin film was manufactured by spray application onto a transparentglass substrate of a dispersion obtained by dispersing 1 to 2[mg] ofrefined SWNT in for example 5[ml] of methanol as dispersion medium.

The results of measurement of the optical absorption characteristic ofthe SWNT thin film which was thus obtained are shown in FIG. 1. Themeasurement was conducted using a spectrophotometer U-4000 (manufacturedby Hitachi Seisakusho). The horizontal axis of this Figure shows theenergy [eV] of the light directed onto the SWNT thin film and thevertical axis shows the absorbance [−] of this SWNT thin film.

As shown in FIG. 1, it can be seen that the SWNT thin film has aplurality of absorption bands in the infra-red region. Also, it can beinferred that this SWNT thin film has semiconductor properties from thefact that it shows an absorption edge in the vicinity of 0.8[eV].

Next, FIG. 2 shows a characteristic obtained by extracting theabsorption band that appears at the lowest energy shown in FIG. 1 (inthis case, in the vicinity of 1[eV]) and converting the horizontal axisto the wavelength [nm] of the light.

As shown in FIG. 2, it was confirmed that the absorption band in thevicinity of about 1[eV] in FIG. 1 is present in a wavelength region of1200 nm to 2000 nm and the absorption peak wavelength is in the vicinityof 1780 nm. It should be noted that, although the absorption peakwavelength of SWNT is in the vicinity of 1780 nm under the conditions ofthis embodiment, it may be envisioned that minute changes in theabsorption peak wavelength may be produced by adjusting the diameter andlength of the SWNT.

(1-4) Measurement of the Saturable Absorption Function of CarbonNanotubes

An evaluation of the saturable absorption function of the SWNT thin filmwas conducted by using the Z-scanning method to measure the relationshipbetween intensity of the incident light and intensity of transmittedlight transmitted through the SWNT thin film, by directing illuminatinglight (laser light) onto the SWNT thin film manufactured by the methodalready described in (1-2).

The measurement device used in the Z-scanning method is showndiagrammatically in FIG. 3. As shown in FIG. 3, in the measurementdevice 10 there are arranged in sequence along the optic axis (Zdirection) of the incident light from the light source 12: a lightsource 12 such as a semiconductor laser, a UV cut-off filter 14, an NDfilter 16, a lens 18 of focal point distance f of 150 mm and aphotodetector 20; the SWNT thin film 15 is arranged between the lens 18and the photodetector 20.

The change in transmittance with intensity of the incident lightdirected onto the SWNT thin film 15 was then measured by moving the SWNTthin film 15 along the leftwards direction (direction of the lightsource 12) in the plane of the drawing of the optic axis (Z axis),taking a position in which the SWNT thin film 15 is offset by about 40mm towards the photodetector 20 from the focal point F of the lens 18 asthe origin X (0: zero).

Laser light of about 1780 nm, which is the absorption peak wavelength ofthe SWNT, was then output using as the light source 12 a regeneratingamplifier titanium sapphire laser provided with an optical parametricamplifier (OPA). Also, measurement was conducted with a pulse width ofthe laser light of 200 fs, a repetition period of 1 kHz, and sixdifferent laser light intensities from the light source 12, namely, 10μW, 20 μW, 30 μW, 50 μW, 100 μW and 300 μW. It should be noted that theamount of light that is incident on the SWNT thin film 15 is greatestwhen this SWNT thin film 15 is positioned at the focal point F anddecreases as the SWNT thin film 15 moves away from the focal point F.Also, as an example, when the laser light intensity from the lightsource 12 is 10 μW, the laser beam diameter at the focal point F wasabout 0.05 mm and the laser light intensity at the focal point F wasabout 637 MW.

FIG. 4 shows the relationship between the various laser lightintensities from the light source 12 and the transmittance when the SWNTthin film 15 was positioned at a position displaced to the vicinity of40 mm (−40 mm) in the leftwards direction in the drawing from the originX (0) i.e. when the SWNT thin film 15 was positioned in the vicinity ofthe focal point F. In FIG. 4, the laser light intensity (laser power)[μW] is shown logarithmically on the horizontal axis and thetransmittance [−] is plotted on the vertical axis. The laser power wasabout 3×10⁻² (3%) at 10 μW, about 9.5×10⁻² (9.5%) at 20 μW, about16.5×10⁻² (16.5%) at 30 μW, about 32×10⁻² (32%) at 50 μW, about 55×10⁻²(55%) at 100 μW and about 80×10⁻² (80%) at 300 μW.

As can also be understood from FIG. 4, the transmittance differs at eachlaser light intensity, depending on the intensity of the incident light,but, in the vicinity of −40 mm, which is in the vicinity of the focalpoint F of the lens 18, optical non-linearity is displayed, in which thetransmittance increases. It was thereby confirmed that the SWNT thinfilm has a saturable absorption function in respect of light (or signallight) of an absorption band in the infra-red region.

However, since, in this embodiment, for example a suitable coating wasnot applied to the SWNT thin film surface, diffusion of the laser lightincident on this SWNT thin film was unavoidable. Accordingly, in thisembodiment, taking into account that the laser light loss produced bythis diffusion is of the order of about 20%, it is considered thatsubstantially 100% of the incident light (laser light) is transmitted ata transmittance of the order of about 80×10⁻² (80%).

<2> Example of a Construction Utilizing the Saturable AbsorptionFunction of Carbon Nanotubes

First of all, an example construction will be described utilizing anoise reducing apparatus in which the noise of signal light is reducedby providing carbon nanotubes constituting a saturable absorber in thetransmission path of the signal light in optical communication.

Furthermore, by combining the saturable absorber that is used as a noisereducing apparatus with an optical amplifier, this can be employed as anoptical isolator in respect of light propagated in the oppositedirection to the signal light. It can also be utilized as a waveformshaper with respect to the signal light.

FIG. 5 is a constructional diagram given in explanation of an EDFA,which is an optical fiber amplifier comprising a noise reductionapparatus for signal light according to the present invention. Also,FIG. 9 is a constructional diagram of a typical prior art EDFA, given inorder to explain the differences in respect of the construction of FIG.5. It should be noted that although FIG. 5 and FIG. 9 are bidirectionalexcitation type EDFAs, there is no restriction to this, and the presentinvention could be suitably applied also to a forward directionexcitation type EDFA or backward direction excitation type EDFA.

Also, although, in this embodiment, the description was given taking anEDFA as an example of an optical fiber amplifier, there is norestriction to this and the present invention could suitably beimplemented for example using a Raman amplifier.

First of all, an example of a typical EDFA construction will bedescribed with reference to FIG. 9.

As shown in FIG. 9, a typical bidirectional excitation type EDFA 30 isprovided between an input 32 and output 42 and comprises opticalcombiners/splitters 34, 34′, exciting light sources 36, 36′, opticalisolators 38, 38′ and an erbium-doped optical fiber (hereinbelow calledEDF) 40. The optical isolators 38, 38′ function as non-reciprocalcircuits that suppress reflected light (optical noise) that ispropagated in the opposite direction to the signal light and that ischiefly generated at the terminals of the input 32 and output 42constituting the connection terminals of the EDFA 30 with other fibers.

An outline of the operation of the bidirectional excitation type EDFA 30is as follows.

First of all, the signal light that is incident from the input 32 iscombined with the exciting light that is emitted from the exciting lightsource 36 in the optical combiner/splitter 34, passes through theoptical isolator 38 and is then amplified by the EDF 40. Unwanted lightsuch as residual excitation light in the optical combiner/splitter 34′and optical isolator 38′ is split from the amplified light and theamplified light is emitted at the output 42 as the desired amplifiedsignal light.

FIG. 5 shows a constructional example of the application of noisereducing apparatus for signal light according to the present inventionto such a prior art bidirectional excitation type EDFA. An example ofembodiment of the present invention is described with reference to FIG.5.

As shown in FIG. 5, a noise reduction apparatus according to thisembodiment is constituted by a saturable absorber using carbonnanotubes. The saturable absorber 15 used in this case is an SWNT thinfilm formed by application onto a transparent glass substrate as alreadydescribed in section (1-3).

In this embodiment, the SWNT thin film on the glass substrate may beformed in a film thickness such as to give a transmittance of about 80%or more in respect of the desired signal light. In this way, undesiredtransmission of optical noise can be effectively reduced withoutimpeding transmission of the desired signal light. In the followingdescription, the noise reduction apparatus may be simply referred to asa saturable absorber.

In this embodiment, this noise reduction apparatus is provided insertedin the transmission path of the signal light of the bidirectionalexcitation type EDFA 50. In this constructional example, a constructionis adopted in which the optical isolator 38′ in the latter stage of theEDF 40 of FIG. 9 is substituted by saturable absorber 15 of carbonnanotubes.

As is well known, the EDFA optically amplifies the signal light of the1500 nm band in the very low loss wavelength zone of a silica fiber byproducing a population inversion in the erbium (Er) by means of excitinglight (exciting wavelength: 980 nm or 1480 nm) supplied by asemiconductor laser. Matching of the wavelength zone (roughly 1200 nm to2000 nm) in which saturable absorption by the SWNT thin film occurs withthe wavelength band of the signal light of the EDFA (1500 nm) cantherefore be achieved.

In this embodiment, as already described in (1-4), the saturableabsorber 15 that replaces the optical isolator 38′ has thecharacteristic of cutting off light of low light intensity (opticalnoise) but of transmitting light of strong light intensity (signallight).

Consequently, it was found that, by skillfully utilizing the differencein optical power of this optical noise and optical power of the signallight and passing these to a saturable absorber 15 constituted of carbonnanotubes, the transmittance of the optical noise can be lowered (infact the optical noise can be substantially cut off) but substantially100% of the signal light can be transmitted.

Thus, if for example the initial light intensity (optical power) of theoptical noise generated in the bidirectional excitation type EDFA 50 isabout 10 μW and, compared with this, the initial light intensity(optical power) of the signal light is of a higher level such as forexample 50 μW or 100 μW or more, the desired function is achieved byconstructing an optical communication system making use of thisdifference of transmittance in accordance with the signal lightintensity. It should be noted that this is merely an example and couldbe suitably altered at will in accordance with the desired set-up.

Next, a detailed description will be given concerning the changes insignal light waveform and optical noise waveform produced by an EDFAequipped with the signal light noise reduction apparatus, with referenceto FIG. 6(A) to (C). It should be noted that FIG. 6(A) to (C) are viewsto explain diagrammatically the changes in signal light waveform andoptical noise waveform and do not necessarily indicate the actualchanges of waveform. Also, the horizontal axis in this Figure representsthe time t (arbitrary units) and the vertical axis represents the signalintensity (optical power) (arbitrary units).

The signal light a is input from the input 32 shown in FIG. 5 to thebidirectional excitation type EDFA 50 together with the optical noise bgenerated accompanying propagation of this signal light a. The lightintensity of this optical noise b is then fairly small in comparisonwith the light intensity of the signal light a (see FIG. 6(A)).

In the stages preceding the saturable absorber 15 of the bidirectionalexcitation type EDFA 50 shown in FIG. 5, the signal light a is amplifiedto produce signal light a′. Also, in the optical amplification stage ofthe signal light a, the initial optical noise b and randomly generatedspontaneous emission or the like are amplified to produce optical noiseb′. The light intensity of this optical noise b′ then becomes of anon-negligible magnitude in comparison with the light intensity of thesignal light a′ (see FIG. 6(B)).

By outputting the signal light a′ and the optical noise b′ through thesaturable absorber 15, whereas substantially 100% of the signal lighta′, which is of large light intensity, is transmitted, producing signallight a″, transmission of the optical noise b′ may be said to be reducedor substantially cut off (see FIG. 6(C)). It should be noted that thewaveform shape of the signal light a″ undergoes waveform shapingcompared with the waveform shape of the signal light a′ (this isdescribed in detail later).

Also, in this embodiment, shifting the absorption peak wavelength of theSWNTs from the vicinity of 1780 nm to the vicinity of 1500 nm is alsodesirable in order to produce an outstanding saturable absorptionfunction of the SWNTs. This can be achieved by adjusting the diameterand length of the SWNTs (reduction of the diameter of the SWNTs isparticularly effective). However, even if the absorption peak wavelengthof the SWNTs and the signal light wavelength are not necessarily madethe same, so long as the signal light wavelength is within the SWNTabsorption wavelength zone, the SWNTs can be practically used.

Also, although, in this embodiment, a construction was adopted in whichthe optical isolator 38′ in FIG. 9 was replaced by the saturableabsorber 15, a construction in which the optical isolator 38 is replacedby the saturable absorber 15 or a construction in which the saturableabsorber 15 is arranged in the latter stage of the bidirectionalexcitation type EDFA 30 can be expected to give the same effects.

Furthermore, the saturable absorber 15 can likewise perform saturableabsorption not merely of the signal light but also of the reflectedlight of this signal light, which is propagated in the oppositedirection to the signal light. Consequently, the saturable absorber 15shown in FIG. 5 can be utilized as an optical isolator that cuts offtransmission of reflected light or can be utilized as an elementproviding noise reduction of the signal light and an optical isolator ofthe reflected light. Consequently, the noise reduction apparatus of thepresent invention can achieve excellent optical transmission with littlenoise degradation, by insertion in the propagation path of the signallight.

In addition, a case where the saturable absorber 15 is employed as awaveform shaper will be described with reference to FIG. 7. In thisFigure, the horizontal axis represents the time t (arbitrary units) andthe vertical axis represents the signal light intensity (optical power)(arbitrary units).

As already described, the portion of large light intensity near thecenter of the light intensity distribution is of high opticaltransmittance while the optical transmittance of the portions of smalllight intensity skirting this portion is lower. Consequently, as shownin FIG. 7, the signal light a′ prior to incidence on the saturableabsorber 15 (corresponding to the signal light a′ in FIG. 6) becomes thesignal light a″, in which the passage of signal light of low lightintensity in this signal light a′ has been cut off by passage throughthe saturable absorber 15.

As a result, the pulsed signal light a″ that is output through thesaturable absorber 15 assumes a waveform in which the leading andtrailing ends of the signal light a′ have been cut off. The pulse widthY of the signal light a″ therefore becomes narrower than the pulse widthx of the signal light a′. Consequently, when the saturable absorber 15shown in FIG. 5 is employed in respect of pulsed signal light, it can beutilized as a waveform shaper that shapes the pulse time width to ashort pulse time width and that shapes the signal light to a steepwaveform, for example to a rectangular shape.

Also, since the noise reduction device of the present inventionutilizing carbon nanotubes constituting a saturable absorber 15 is anoptical device of long life having resistance to optical damage andmechanical damage and resistance to water it may be expected to find awide range of utilization in the field of optical communication.

The conditions etc of this embodiment of the present invention are notrestricted to the combinations described above. The present inventioncan therefore be applied by appropriately combining conditions in anysuitable desired stages.

For example, the saturable absorption function of a noise reductiondevice according to the present invention can be applied to (signal)light from any suitable generating source without being restricted tosignal light from an optical amplifier. For example, although, in theembodiment described above, the case was described in which the noisereduction apparatus employing a saturable absorber was applied to thefield of optical communication, it could also be suitably applied to thefield of semiconductor devices.

That is, in a construction in which the resonator is omitted from alaser and a semiconductor device, for example a semiconductorconstituting an optical amplification medium is employed, as shown inFIG. 8(A), unwanted noise, generated from a semiconductor opticalamplifier 60, that tends to cause a lowering of product reliability, canbe reduced or excluded by performing saturable absorption in respect ofthe emitted light by inserting a noise reduction apparatus according tothe present invention in the optical path of this light emitted fromthis semiconductor optical amplifier 60. Also, effects such as reductionor exclusion of noise as described above may be anticipated by insertinga noise reduction apparatus according to the present invention in theoptical path of this emitted light also in the case of the semiconductorlaser 62.

Also, a construction could be adopted in which, as shown in FIG. 8(B),carbon nanotubes constituting a saturable absorber 15 are provided asrepeaters for each adjacent optical fiber amplifier in a case in which aplurality of optical fiber amplifiers, such as for example threeconsecutive optical fiber amplifiers, (for example bidirectionalexcitation type EDFAs 50 as shown in FIG. 5) are arranged in thetransmission path of the signal light. In this case, saturableabsorption is performed in respect of each of the amplified beamsemitted from the consecutive optical fiber amplifiers 50, so unwantedASE can be more efficiently cut-off (reduced).

It should be noted that the number of consecutive optical fiberamplifiers is not restricted to three as described above. For example,in fact, increased transmission distance span while compensating forattenuation of the signal light can be achieved by providing one opticalfiber amplifier approximately at each 80 km of the optical fiber. Ifthis is done, amplification of the optical noise is repeated togetherwith the amplification of the signal light, so the effect of opticalnoise increases to such a degree that it cannot be neglected and, as aresult, prevents accurate propagation of the signal light.

Accordingly, as described above, even simply by adopting a constructionin which noise reducing apparatus according to the present inventioni.e. carbon nanotubes constituting saturable absorbers together with asuccession of optical filters are provided so that for example theoptical noise level is reduced by about for example 10% for each set ofcarbon nanotubes, the action is obtained of reducing the effect ofoptical noise in an extremely effective manner at for example a 10,000km base point and so of suppressing the drop in S/N ratio.

It should be noted that the glass substrate is not restricted in any wayto a parallel planar plate and a glass substrate of any suitable shapecould be adopted in accordance with the application or design. Also,instead of the glass substrate, a plastics substrate or the like couldbe employed as the transparent optical material.

Industrial Applicability

As will be clear from the above description, according to the presentinvention, the saturable absorption function provided by carbonnanotubes can be utilized in the optical communication field as a noisereducing apparatus for signal light, that transmits signal light of highlight intensity but yet cuts off transmission of for example ASE, whichis of low signal light intensity. As a result, a reduction in forexample ASE can be achieved, so even greater increases in the span ofthe transmission distance can be achieved.

1. A noise reduction apparatus for signal light is arranged in thetransmission path of this signal light in order to reduce noise of thesignal light in optical communication and a carbon nanotube is used as asaturable absorber.
 2. The noise reduction apparatus for signal lightaccording to claim 1, wherein said carbon nanotube has opticalnon-linearity.
 3. The noise reduction apparatus for signal lightaccording to claim 1, wherein said saturable absorber has a function asan optical isolator in respect of light propagated in the oppositedirection to said signal light by combination with an optical amplifier.4. The noise reduction apparatus for signal light according to claim 1,wherein said saturable absorber has the function of a waveform shaper inrespect of said signal light.
 5. The noise reduction apparatus forsignal light according to claim 1, wherein the wavelength zone of saidsaturable absorber that is capable of saturable absorption is at least1200 nm but no more than 2000 nm.
 6. The noise reduction apparatus forsignal light according to claim 1, wherein said signal light is signallight that is emitted from an optical fiber amplifier.
 7. The noisereduction apparatus for signal light according to claim 6, wherein saidoptical fiber amplifier is an erbium-doped optical fiber amplifier. 8.The noise reduction apparatus for signal light according to claim 1,wherein said signal light is signal light emitted from a semiconductoroptical amplifier.
 9. The noise reduction apparatus for signal lightaccording to claim 1, wherein said signal light is signal light emittedfrom a semiconductor laser.
 10. The noise reduction apparatus for signallight according to claim 1, wherein said saturable absorber is providedas a repeater between each adjacent said optical fiber amplifier when aplurality of said optical fiber amplifier stages are providedconsecutively in said transmission path.
 11. The noise reductionapparatus for signal light according to claim 1, wherein said carbonnanotube is either or both of a single-wall carbon nanotube ormulti-wall carbon nanotube.
 12. The noise reduction apparatus for signallight according to claim 1, wherein said saturable absorber is providedon a transparent optical component.
 13. A method of noise reduction ofsignal light, wherein a carbon nanotube is arranged as a saturableabsorber in the transmission path of signal light in opticalcommunication, noise of the signal light being reduced by means of thesaturable absorber.